The present invention generally relates to a high power light source. More specifically, the present invention is directed to a light source comprising a plurality of laser diode modules having high optical power and arranged in high density.
Normally, laser diode modules are employed as signal light sources of optical fiber communications, especially, signal light sources of main trunk systems/CATV systems, and light-excitation light sources of fiber amplifiers. In such a laser diode module, a Peltier-effect element is built therein, and various optical components and various electronic components are arranged on a metal substrate mounted on the Peltier-effect element in order to realize high optical power and stable operations of the laser diode module. The optical components are a laser diode chip, a photodiode chip, a lens, and the like whereas, the electronic components are a thermistor element, an inductor, a resistor, and the like.
It should be noted that the above-explained Peltier-effect element is a thermocouple semiconductor. In the case that the Peltier-effect element is made from a p-type semiconductor, when a DC current is supplied to the Peltier-effect element, heat is moved along the current flowing direction. In the case that the Peltier-effect element is made from an n-type semiconductor, when a DC current is supplied thereto, heat is moved along a direction opposite to the current flowing direction, so that a temperature difference is produced between both ends of the thermocouple semiconductor. In a cooling system using such a Peltier-effect element, a low-temperature side thereof is used for cooling, and a high-temperature side thereof is used for heat dissipation, while utilizing the above-explained temperature difference.
In the laser diode module, a temperature of the above-explained laser diode chip is detected by the thermistor element positioned in the vicinity of the laser diode chip. The laser diode module includes the following structure which is capable of keeping the temperature of the laser diode chip constant. That is, the thus detected value of the temperature is fed back so as to drive the Peltier-effect element, so that the entire metal substrate where the laser diode chip is arranged is cooled.
FIG. 5 depicts a conventional laser diode module. FIG. 5 is a sectional view for schematically showing the conventional laser diode module. As shown in FIG. 5, the laser diode module includes a mount 113 for mounting thereon both a laser diode chip 111 and a heat sink 112, a chip carrier 115 for mounting thereon a monitoring photodiode chip 114, a lens holder 116, a metal substrate 110a for mounting thereon a resistor, an inductor, and a circuit board (not shown); and a Peltier-effect element 117. The Peltier-effect element 117 is fixed on a heat dissipating plate 118 of a package by metal solder. It should also be noted that ceramics plates 119A and 119B are arranged on upper and lower portions of a Peltier-effect element 117.
FIG. 6 is a sectional view for showing the laser diode module, taken along a line A to Axe2x80x2 in FIG. 5. As shown in FIG. 6, as an essential portion of the laser diode module, a thermistor 121 and the laser diode chip 111 are mounted on the heat sink 112. As a metal solder used to adhere the Peltier-effect element 117 to the metal substrate 110a, soft solder 122 is employed in order to relax a thermal expansion difference between the two members.
The above-explained metal substrate is in general made of a single material such as copper tungsten (CuW: weight distribution ratio of copper is 10% to 30%). When the metal substrate is adhered to the Peltier-effect element, low-temperature soft solder such as indium tin (InSn) is employed so as to relax the thermal expansion difference between the two materials.
However, recently, more severe requests are made with respect to both the cooling capability of the laser diode module, and the temperature environmental reliability (namely, capability of maintaining normal functions under the condition even when temperature varies).
At first, in order to improve the cooling capability, the size of the Peltier-effect element should be made large, and also the metal substrate mounted on the upper portion thereof must be made from the high heat transfer material. Since the temperature adjusting time (namely, time duration until target temperature is reached) is reduced due to improvements in the cooling capability of the Peltier-effect element, the temperature stress given to the metal substrate mounted on the Peltier-effect element is also increased. As a result, the adverse influence given by the difference of the heat expansion coefficients between the Peltier-effect element and the metal substrate is increased. As a result, there is such a problem that cracks and exfoliation will occur, because the soft solder used to adhere the both members is slid. Moreover, since the soldering creep phenomenon which is specific to the soft solder becomes apparent, such a low-temperature hard solder as bismuth tin (BiSn) must be employed as the solder for adhering the Peltier-effect element to the metal substrate.
To solve the above-explained problem, Japanese Patent Provisional Publication No. Hei 10-200208 discloses a semiconductor laser module including a metal substrate made of two different kinds of metal materials. FIG. 7 schematically shows a conceptional structure of the semiconductor laser module. As shown in FIG. 7A, the semiconductor laser module is manufactured as follows: a metal substrate 210 is adhered to a Peltier-effect element 207 with ceramics boards 209A and 209B mounted on upper and lower surfaces thereof by using hard solder 212. An LD chip 201 and a thermistor 211 are mounted on the metal substrate 210 through a heat sink 202 and a sub-mount 203 together with a lens of an optical system. The thermistor 211 is employed so as to keep the temperature of the LD chip 201 constant.
The metal substrate 210 is adhered onto the upper surface of the Peltier-effect element 207 in such manner that a heat flow derived from the LD chip 201 directed to the Peltier-effect element 207 is in perpendicular thereto. In particular, the metal substrate 210 is formed in such a manner that a first metal member 213 is arranged at a center portion of the substrate including a portion located directly below the LD 201, and a second metal member 214 is arranged so as to surround the first metal member. Furthermore, as depicted in FIG. 7B, the metal substrate 210 is manufactured in such a manner that the first metal member 213 is formed by such a metal member having a large heat conductivity, whereas the second metal member 214 is made of such a metal member having a heat expansion coefficient smaller than that of the first metal member 213.
In other words, it is expected that since the above-explained metal substrate 210 is employed, the heat expansion of the entire metal substrate can be reduced, the heat condution thereof can be improved so as to increase the cooling performance. At the same time, it is expected that reliability of the Peltier-effect element is improved.
It should also be noted that in general, a plurality of laser diode modules functioning as a light output source are mounted on either the light-excitation light source or the optical-signal light source. A laser diode module is combined with other optical components so as to be used in an optical amplifier.
In accordance with the above-explained prior art, it is so expected that the cooling performance of the Peltier-effect element may be improved and also the reliability of the Peltier-effect element may be increased in each of the laser diode modules. However, in the case that the respective laser diode modules output higher optical power, and also a large number of such high-power laser diode modules are arranged in high density to be driven, the resulting heat generated from the high-power laser diode modules arranged in high density could not be properly treated by merely increasing the heat conducting property of the metal substrate which is arranged between the chip and the Peltier-effect element, or by merely reducing the difference in the heat expansion coefficient. As a result, there is another problem that the functions of the laser diode module would be damaged.
More specifically, since the size of each of these laser diode modules per se is small, but a high density heat generator, when a plurality of these laser diode modules are required to be mounted as either the light-excitation light source or the optical-signal light source, it is practically difficult to dissipate heat from the laser diode modules. On the other hand, further improvements in high light output power are needed in either the light-excitation light source or the light-signal light source. In the conventional method, there is a limitation in the cooling effect achieved by the Peltier-effect element of the laser diode module. As a result, the laser diode modules could be used only under such a condition that the performance of the semiconductor element remains far below 100%.
Furthermore, even when the optical power of the laser diode module is increased in response to needs of the market, there is a strong need that the power consumption caused by excitation of both the Peltier-effect element and the semiconductor element is required to be kept lower than that of the conventional art. Therefore, the heat dissipation property within the light source may become very important.
In addition to the laser diode module, another request is made of treating the heat generated by the laser diode module control board equipped with another heat generating element (for example, CPU) for controlling the laser diode module.
As previously explained, developments of either a light-excitation light source or an optical-signal light source, which is mounted on a heat sink having excellent heat dissipation, are strongly expected.
There is provided a light source having laser diode modules of the invention comprising a plurality of laser diode modules being arranged in high density, each of said laser diode modules having an optical power of at least 300 mW. More specifically, in a light source having laser diode modules of the invention, the light source comprises:
a plurality of laser diode modules, each of which includes a metal substrate and a Peltier-effect element thermally connected to said metal substrate, said metal substrate mounting thereon a laser diode chip and an optical appliance; and
a mounting portion comprising one plate type heat pipe, on which said plurality of laser diode modules are mounted.